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SODIUM CHLORIDE                                    99


NaCl = 58-46 (58-5)                                   *

This is sold in various degrees of purity, the most common impurities
being: potassium and magnesium chlorides, sodium, calcium and mag-
nesium sulphates, insoluble substances (sand, clay) and sometimes small
quantities of bromides, iodides, borates and lithium salts

Analysis includes, therefore, tests for the above and any other impurities
(i—6) and, if the exact composition of the salt is to be known, certain quan-
titative determinations (7).

1.  Solubility and Various Impurities.—I gram in 10 c.c. of water
should give a clear, neutral solution.

The i : 10 solution is tested with hydrogen sulphide (heavy metals),
ammonium sulphide (iron, zinc], ammonium chloride, ammonia and ammon-
ium oxalate (lime) and, after removal of any lime present, with sodium
phosphate (magnesium).

2.  Potassium Salts.—These are detected in the flame through cobalt
glass, or by dissolving i gram of substance in a little water, adding platinic
chloride, evaporating on a steam-bath, taking up with 50 c.c. of 80% alcohol,
any yellow, Crystalline precipitate formed immediately or after some hours
being observed.

3.  Lithium Salts.—A few grams of the finely powdered salt are moist-
ened with 90% alcohol and filtered, the filtrate being examined in the flame
(through cobalt glass) or better through a spectroscope.

4.  Sulphates.—The   i : 10 solution is treated with barium chloride
in the hot and the liquid examined after standing.

5.  Iodides, Bromides.—5 grams are moistened with a little water
and filtered ;  to the filtrate are added a crystal of sodiam nitrite and acetic
acid, the whole being then shaken with carbon disulphide, which becomes
violet or yellowish in presence of iodides or bromides respectively.

6.  Boric Acid.—25 grams are heated, with occasional shaking, with
about 50 c.c. of 95% alcohol acidified with hydrochloric acid and filtered,
the filter being washed with a little alcohol and the filtrate rendered alkaline
with sodium hydroxide and evaporated on a steam-bath.    The residue is
taken up with a little dilute hydrochloric acid and the solution tested with
turmeric paper (better, paper immersed in 0-1% alcoholic turmeric solution),
which is then dried at 100° (reddening).

7.  Quantitative Determinations.—(a) MOISTURE.   5 grams of the
salt are heated for 3-4 hours in a dry, tared conical flask of about 250 c.c.
capacity, with a funnel inserted in the neck, on a sand-bath at 140-150°.
The flask is then allowed to cool in the air on a marble slab and reweighed.

(b)  INSOLUBLE SUBSTANCES.    10 grams   of   the  salt are dissolved in
water, the solution being filtered through a filter dried at 100° and tared
The insoluble matter is washed with nearly half a litre of water, dried at
100° and weighed.    The filtrate is made up to 500 c.c. and used for the
following determinations.

(c)  CHLOKINE.   50 c.c. of this solution (= i gram of salt) are titrated
with silver nitrate in the usual way. • i c.c. N/io-AgNOg = 0-355% Cl.RIDE.   With ammonia  soda, 20 c.c. (=2 grams of